So. I have Sims 2. Yeah, it can be racy, and they can be evil, but do you really think your child is going to feel this is okay if they see simulated characters doing this? If you feel like your child is mature and ready to handle some video games then let them play. Sims 3 is a much better alternative to, say, Grand Theft Auto, where violence is a key part of the game. In Sims 3, you can be naughty or nice.
As for the lust factor, make sure your child knows about this. If its a mystery, they'll be more likely to try to do more stuff. The mystery is the fun of the thing, so make sure everything is open. You can check up on them if you want to- you can take it away, but my feeling is that most mature middle-schoolers can handle this. I rate it as appropriate for 12+, iffy for 10-11, and inapropriate for all immature people.